In connection with air cylinders or other forms of fluid power motors, it is quite conventional to utilize rubber or composition seal rings which have a U-form or U-cup configuration and are adapted to be placed in peripheral grooves extending around the circumference of the piston member. The U-shaped seal rings are positioned within their respective grooves such that one side or one lip of the U will move back and forth against the inside wall of the cylinder while the other side rests in the bottom portion of a peripheral groove. This type of seal, as well as with most types of seal rings, is never completely leakproof in that a small amount of air, or pressurizing fluid medium, can pass around the seals such that the zone between the seals and around the central portion of the piston can be pressurized to some extent. Under most operating conditions, this trapped pressure will be released by a slight deflection of a seal lip away from the inside of the cylinder bore to allow excessive pressure to escape. However, under certain dynamic conditions of cylinder operation, the seal lips can be compressed by the escaping pressure to a point where they contact a groove side wall in the piston member and thus form a closed chamber between the lips of the U-form seal member and the contacts of the ends of the lips with the groove wall. Thus, when the exhausted or low pressure end of the cylinder is rapidly repressurized, there will exist sufficient pressure differential between the low pressure air trapped in the interior of the seal and the high pressure air surrounding the ends of the piston member to compress the outer lip of the seal. This force in turn can be sufficient to maintain the seal ring lips in a compressed, deflected position and thus allow free passage of air past the seal ring and the piston member itself. Also, inasmuch as a U-cup type of seal is positioned in a piston groove in a manner to be unidirectional in action, a second piston seal on the other end of the piston member will readily pass air from the high pressure side of the piston to the low pressure side thereof and allow both chambers of the air cylinder to reach the same pressure. Actually, under this equal pressure condition the piston will stall within the cylinder and cannot produce a force on the piston rod.
The aforedescribed problem with respect to the periodic failure of U-cup seal rings in piston members for fluid power motors is generally known to the industry and various methods of trying to overcome the problem have been carried out. Typically, there are two basic methods of preventing this type of seal failure. In connection with one method of correction, there are mechanical support means that can be placed between the lips of the U-form seal member and against the side of a piston groove such that the seal lips, even though deformed by peripheral fluid pressure, cannot contact the piston groove wall to form a closed chamber.
With respect to another method of preventing seal failure, there has been means provided to vent the interior space between the lips of the seal member such that there will always be an equilization of pressures between the interior of the cylinder and the interior space between the lips of the seal rings in order to allow the lips thereof to spring back into proper contact with the cylinder wall. In some instances, there have been small holes drilled through the ends of the piston to reach the side wall portion of the groove for the seal member such that there is a venting provided for the interior zone between the lips of the seal member and the resulting prevention of a trapped low pressure zone. However, the drilling of holes through a piston member is a costly manufacturing operation and in addition will leave burrs on the inside of the groove which are difficult to remove. Any remaining burrs on the inside wall of a piston groove will interfere with the action of the seal member and, additionally, can cut the ends of the lips and cause excessive rapid wear. In another arrangement, there has been the provision of a smaller diameter end portion for the piston such that there is no circumferential wall portion for an outer lip of a seal member to become sealed against and thus provide for the trapping of low pressure air. However, in connection with this type of construction, it has been found that the friction or drag of a seal lip against the inside of the cylinder wall can totally dislodge a seal ring from the piston groove to in turn result in the total loss of cylinder operation.
In view of the problems of obtaining a suitable vent arrangement for seal members in the grooves of a piston member, it is a principal object of the present invention to provide a simplified design and construction which provides at least one notch means on each of the peripheral end portions of a power cylinder piston member, with such notch means being to a depth that a U-cup seal can be vented between the lip portions thereof and prevented from remaining in a compressed or collapsed condition with the ends of the lip members being held tightly against the wall of the piston groove.
In one embodiment, the present invention provides in connection with a fluid power cylinder having a movable piston with peripheral grooves for U-form seal members to effect a seal with the cylinder wall, the improved construction to prevent low pressure fluid from being trapped in a hollow between the lips of the U-seal member which comprises, providing a notch means in the peripheral end portions of the piston member to a depth reaching a mid portion of the seal holding grooves whereby to insure the venting of the interior portions of the seal members held within the piston grooves.
Actually, various shapes may be provided for the notch means at the edge of a piston member; however, in order to insure rapid and adequate venting of the interior of a U-form seal member within a groove, there may be opposing notch means on an end portion of the piston member such that venting can actually take place from two zones for the space between the lips of a seal member.
Still further, for purposes of manufacture and for the assembly of piston members with piston rod members, it has been found to be of particular advantage to have the notch means milled or broached as flats across the end portions of the piston. In other words, milled or broached flats will be diametrically opposite of each other and will form two chord-like cuts from the peripheral end portions of the piston member, with each cut being carried back into a piston seal groove such that there will be venting from the interior of the groove and from the chamber normally formed between the lips of the seal and the wall of the piston groove. By providing two opposing chord-like flat sections, there are resulting flats which can be used as clamping surfaces for a wrench or for vise gripping means. During the manufacture of the flat notches, any burrs that may be produced in forming the flats will be on an exposed accessible edge and thus easily removed in a rapid assembly line procedure or by a tumbling operation. The flat cuts will extend from the outer circumference or periphery of the piston to a depth sufficient to reach a mid portion of the groove wall and thus permit adequate venting from the space between the lips of the seal member; however, the remainder of the groove wall will be of full depth from the outer periphery of the piston to the small diameter, bottom portion of the peripheral seal holding groove.